The present invention relates to a colour cathode ray tube, particularly, but not exclusively, to a blue light emitting cathode ray tube used in a projection television (PTV) system.
Colour projection television systems normally comprise three cathode ray tubes emitting blue, green and red light, respectively. This light is mixed to produce a coloured image at a viewing screen. In forming the coloured image a number of factors have to be taken into account, these factors include chromaticity, brightness, efficiency, deterioration of the radiant efficiency of the phosphor under electron bombardment, thermal quenching at high operating temperatures, and the construction of an operative system embodying the projection television cathode ray tubes. For good colour reproduction it is important to put the chromaticities of the primary emission colours of the phosphors in the display tube or tubes (in PTV systems) as near to the corners of the CIE chromaticity diagram as possible because it is impossible to produce colours outside the triangle formed by the chromaticities of the primaries. In practical systems, the system designers endeavour to conform to the industry standards for chromaticity. There is a reference point in the known CIE colour triangle referred to as the white-D. It is desirable that the white-D luminance is as high as possible consistent with good chromaticity of the primaries which fulfil the industry standard specification. The white-D brightness of PTV systems at the moment is determined by the output of the blue emitting component ZnS:Ag (silver activated zinc sulphide). The white-D capability (or figure of merit) of blue emitting phosphors is given by ##EQU1## where
.eta..sub.CR is the energy efficiency of the phosphor under cathode-ray (CR) excitation,
L is the lumen equivalent of the spectral emission,
y is the y-coordinate of the chromaticity, and
.eta..sub.L is the so-called lumen efficiency of the phosphor (Lumens out/Watt input).
The main disadvantage of ZnS:AG is that its efficiency decreases with increasing beam current. In consequence the efficiency of ZnS:Ag at high beam currents is low and therefore limits the white-D luminance. Although other blue light emitting phosphors are known, the chromaticities of their emission are not acceptable because the y colour coordinate is either too high, which means that it is not possible to obtain a full range of colours, or too low so that the amount of blue light required is too critical to adjust and operate a PTV system.
An object of the present invention is to alter considerably the chromaticity of phosphors, especially the blue phosphor, as viewed in projection cathode ray tubes, without decreasing the white-D capability.